1. Field of the Invention
The present invention relates to system and method used to measure the condensate and the temperature at the same location on a sensor and more particularly pertains to a new improvement for high precision ultrasonic chilled surface dew point hygrometry for measuring the density of the condensation and the temperature at the same location on the surface of the sensor. This invention was made with government support under DMI9531504 awarded by the National Science Foundation and the government has certain rights in this invention.
2. Description of the Prior Art
The use of a system and method used to measure the condensate and the temperature at the same location on a sensor is known in the prior art. More specifically, system and method used to measure the condensate and the temperature at the same location on a sensor heretofore devised and utilized are known to consist basically of familiar, expected and obvious structural configurations, notwithstanding the myriad of designs encompassed by the crowded prior art which have been developed for the fulfillment of countless objectives and requirements.
Known prior art includes U.S. Pat. No. 5,739,416; U.S. Pat. No. 4,876,889; U.S. Pat. No. 4,088,969; U.S. Pat. No. 4,345,455; U.S. Pat. No. 5,139,344; and U.S. Pat. No. 4,948,263.
Chilled mirror hygrometry is the most widely used form of chilled surface dew point measurements. There are several instruments available that use this technique combined with continuous control of the dew layer such as the Edgetech (formerly EG&G) series 2000 Models and General Eastern Models Hygro-M1 and M2. These instruments provide continuous dew point measurements by using optical reflection to detect and control the condensate while using a resistive temperature device (RTD) to measure the temperature. They are limited to an accuracy and resolution of +/-0.2 C, are sensitive to mirror contamination, become unstable as the dew deposit freezes, and cannot determine the phase of the deposit. Protometer PLC markets a chilled mirror instrument, the System 996, where the mirror is alternately cycled between dry and wet, and the dew point temperature is measured as the mirror enters the wet phase. This cycling reduces contamination effects and avoids the frost point transition since the dew does not have time to freeze in the temperature range that the instrument operates, but with the trade-offs of reduced accuracy and non-continuous measurements.
Surface acoustic waves (SAWs) to detect condensation and control condensation density on the surface of the sensor were first described in U.S. Pat. No. 4,378,168 issued to Kuisma et al. This design uses SAW attenuation in a single sensor (delay line) configuration. It further uses a thermoelement attached to the sensor surface to measure the surface temperature. The invention described in U.S. Pat. No. 5,364,185 issued to VanZandt et al. claims to vary from current chilled mirror instruments in two respects: 1) It uses interdigital transducers (IDTs) to detect water vapor using changes in capacitance or the resonant frequency of a surface acoustic wave resonator; and 2) a parametric approach of dew point detection in which a change in transducer output is measured as a function of a specific thermodynamic quantity. Specifically they use a peak in the second derivative of the moisture signal versus temperature to indicate the dew point which they suggest is relatively immune to surface contamination. The temperature can be measured with a temperature sensor that is located alongside of the SAW resonator. This system can also be less sensitive to surface contamination than optical techniques. SAWs have been used by other investigators such as Hauden et al. (see D. flauden, G. Jaillet and R. Coquerel, "Temperature Sensor Using SAW Delay Line," IEEE Ultrasonics Symposium, 1981, p. 148-151) and Neumeister et al. (see J. Neumeister, R. Thum and E. Luder, "A SAW Delay-line Oscillator as a High-resolution Temperature Sensor, Sensors and Actuators A21A23 (1990) 670-672) to measure temperature, while Mingfang and Haiguo (see L. Mingfang and L. Haiguo, "SAW temperature and humidity sensor with high resolution," Sensors and Actuators B, 12 (1993) 53-56) used a dual delay line SAW sensor to measure both temperature and humidity by coating one delay line with a polymer. Hoummady et al. (see M. Hoummady et al., "Surface acoustic wave (SAW) dew point sensor: application to dew point hygrometry," Sensors and Actuators B, 26-27(1995) 315-317) used SAWs to provide non-continuous dew point measurements by cooling a single delay line SAW device until dew formed and then measuring the dew point temperature as a discontinuity in the frequency versus temperature curve of the SAW device. However, no one has developed a SAW or other acoustic sensor method that can measure temperature while continuously maintaining a constant condensation density using a dual sensor approach.
The most significant limitations of this current art are some or all of the following: 1) temperature measurement inaccuracy due to temperature gradients between the condensate location and the RTD, and self heating of the RTD; 2) limited resolution; 3) instability during the frost point transition; 4) inability to determine the phase of the condensate; 5) non-continuous measurements.
While these devices fulfill their respective, particular objectives and requirements, the aforementioned patents do not disclose a new improvement for high precision ultrasonic chilled surface dew point hygrometry. The inventive device includes a piezoelectric sensor having a surface comprising a piezoelectric substrate, and further having a conventional thermoelectric cooler connected thereto and a plurality of spaced heat sinks conventionally depending from the surface. Conventional acoustic devices are used to propagate surface waves across the surface of the sensor. A series of surface acoustic waves are used to measure the density of the condensation at a particular location on the surface of the sensor. A conventional wave detection device such as a phase detector detects the waves as to its velocity and amplitude and passes this information onto a computer microprocessor which is programmed to measure and control the density of the condensate located along the path of the waves across the surface of the sensor. A series of surface skimming bulk waves are preferably propagated across the surface of the sensor at generally an angle to the surface acoustic waves so as to measure temperature of the surface of the sensor without interfering with the surface acoustic waves. Other temperature measuring techniques can also be used, one of which includes using a conventional integrated resistive temperature device disposed in a nonobvious configuration so as to not to interfere with the surface acoustic waves on the surface of the sensor.
In these respects, the improvement for high precision ultrasonic chilled surface dew point hygrometry according to the present invention substantially departs from the conventional concepts and designs of the prior art, and in so doing provides an apparatus primarily developed for the measuring the density of condensation and the temperature at the same location on the surface of the sensor.